Bonus Heros

How many bonus heroes you select has no bearing on the odds of of getting a bonus hero.  It only affects the odds that you'll pull a specific hero.  In other words, if you select one bonus hero per tier, then you have 100% chance of that particular hero being the bonus hero when you're lucky enough to get one. If you select 4 per tier, then you have 25% (each) of getting a particular character, but the total odds still stay the same.  Just remember that when you earn a bonus hero it's only for the tier that you won it in.  So, if you pull a 3* cover and it gives you a bonus, it can only be a 3* bonus.

tl;dr, your overall odd don't change no matter how many you pick.  The odds of getting a specific bonus hero is 100/#bonus heroes selected

I set one character at each star tier as my bonus hero. It can be useful for building up an under-covered character quickly, but I'll also set characters I already have champed to get a specific cover off a champ reward (this is especially useful at the 4-star tier).

I agree w/ the strategy of having a single bonus hero for each tier.  Like w/ everything in the game, you should have a plan how to best take advantage of a specific game related characteristic.  Between dilution and RNG, having a set (however infrequent) chance at a character you want/need is very valuable to help cover characters that you want to roster. 

I missed most of the 4* "meta" character releases.  I'm still WAY behind on G4mora & RNG, but I've been able to fully cover and champ Rogue, along w/ getting Carnage and Medusa fully covered.  A couple of months ago, I laughed at the idea of getting them covered, but w/ the help of bonus heros, I've come quite a long ways.  Having a specific 3* bonus hero can help get 4* progression rewards, and obviously having that 4* as a bonus hero helps too.  I'm not in 5* land, but I have set my 5* bonus hero to Thor and it's helped me add 2 covers I wouldn't have had otherwise and now I have at least 1 cover in all 3 colors.  

I have 1 BH per tier at 5 & 4 tier with the goal of getting covers for those characters. At 3 as I am not currently trying to champ anyone I am focusing on getting the good rewards so I rotate based upon who is near what - mostly this is for LT's but may change if there is a nice chunk of CP or a 4* cover needed. When I get a few more covers for my dupe Patch then I will BH to get him re-champed.

yes, latest and classic status only affects the draw odds from those stores, it's not a fundamental distinction. So you can set the latest characters to BH, and then pull out of classic if you want or vice-versa. I think a lot of people, myself included, spend CP in classics because every 5th pull is essentially "free" compared to using it in latest. Your odds of getting any specific 5* are dismal, but your odds of getting a 5* at all are the same.

Saving up (hoarding) until you have enough pulls to cover the odds in latest is still the generally accepted way to transition into being a 5* player, but it's a huge time commitment that a lot of people aren't really willing to undertake to bank up that many LTs and CP (is it 400 pulls now with cover saves instead of customer service 1:1 swaps? Used to be 300 was the magic number of pulls). 

Just don't rely on bonus heroes for 5* covers.  After pulling several hundred of the latest legend tokens I think I have only received one or two bonus 5* heroes ever, they are very rare to get in that tier.  

I have used bonus heroes in 3* tier to get to LL tokens faster at the different levels - 266, 257, 237?, etc.  in order to get more draws for 5* LL that I want, like JJ right now before they rotate out of LL.

I do the same thing as DAZ; set one BH at the 4* and 5* tier at a time, just depending on who you're working on.  In the 3* tier, I have everybody who's one level away from yielding an LT bonused. Once I hit that level, I un-bonus them. It's just a way to maximize the number of LTs incoming.